Canon have been the top-seller for ages, although recently overtaken by their really major rival, Nikon. Sony are establishing themselves in the SLR business having bought Konica-Minolta. Consequently, both Canon and Nikon have much clearer product lines and offer a cohesive upgrade path from a beginner SLR like the XTi right through to top-end pro stuff.
I can't really comment on which is better out of Canon & Nikon. I'm a Canon shooter myself, and since I have in excess of £8k invested in Canon gear, I'm simply never going to switch to another brand. You need to pick a brand now that you'll live with pretty much forever.
With regard to the two cameras that you've shortlisted, I would choose the Canon, but then it is what I know! One key difference between Canon & Sony is that with Canon gear, the Image Stabiliser kit is installed in the lenses, whereas the Sony IS kit is in the body. Whilst this means that you have to buy the IS facility in all your stabilised lenses, it does also mean that the IS is optimised for that lens, where a single body cannot hope to cope with the differing IS requirements of, say, a 17-55mm walkabout lens and a 600mm super-telephoto.
Canon have announced the replacement for the XTi, called XSi, I think. It should mean that there's a deal to be done on the XTi.